The applicable words in Gen. 22:12 in Hebrew are "For now I know..." Either God didn't know before that moment, or He lied when He made that statement. We know the latter is not possible.
The quoted text is part of a (much) larger whole (Genesis 22:1-18), which seems primarily concerned with Abraham's inner transformation, rather than God's (fore)knowledge. The ancient patriarch had the potential to become a truly devoted follower of God, and God obviously sensed that. However, merely possessing a certain potential is not quite the same as achieving it, and having a specific calling is not quite the same as living up to it. As he was ready to sacrifice that which he held most dear to please God, he fulfilled his true potential, and succeeded to live up to his divine calling, of becoming the father of all believers. This, to me, seems the natural interpretation of the words now I know, namely now you have finally proven yourself to Me and to all posterity, that you are truly worthy of receiving My blessings.
From this passage, it's evident that God didn't know for certain what Abraham's decision would be, until He saw that Abraham was ready to slay his own son. Then, He knew. But doesn't God know everything, because "He's God"? Doesn't He know what decisions we will make, before we make them? Certainly God knows what we're LIKELY to do, or decide, in any given situation, based on what He's seen us do before, and our basic attitude. But we can think we're going to do something, and then change our mind, perhaps many times, vacillating back and forth, agonizing, pondering, weighing the merits of doing versus not doing. I think it's safe to say, God has to see, before He's truly convinced.
First of all God is "omniscient" which means He knows everything past, present and future. Proverbs 15:3 or Psalm 44:21. Secondily, God does not lie because He can't lie due to His nature/essence. Numbers 23:19.
Now, at Genesis 22:11 we have the angel of the Lord calling out from heaven saying to Abraham at vs12, "Do not stretch your hand against the lad, and do nothing to him; for (or because) now I know that you fear God, since you have not withheld your son, your only son from Me."
I am totally (Biblically) convinced that the angel of the Lord is the preincarnate Jesus Christ and he is speaking in the third person. Job 1:8 and at Job 2:3. "And the Lord said to Satan, Have you considered My servan Job? For there is no one like him on the earth, a blameless and upright man fearing God, and turning away from evil." Other examples would include 1 Samuel 2:30, Numbers 14:28 and many others.
Also, you have the angel of the Lord calling out to Abraham a second time from heaven at Genesis 22:15 and he says at vs16, "and said, By Myself I have sworn, declares the Lord because you have done this thing, and have not withheld your son, your only son."
It should also be noted that when God says, "I know that you fear God" can mean that God recognized Abraham as a God fearer. And btw God in numerous places made statements like at Genesis 3:9 where He ask Adam, "Where are you?" At Genesis 4:9, He ask Cain, "where is Abel your brother?" None of these types of questions mean that God is lacking any sort of knowledge.
I want to add that if the angel of the Lord is truly an actual angel why does God have him call out from heaven two times at Genesis 22:11,15? God had no problem calling out from heaven Himself at Exodus 20:22, "Then the Lord said to Moses, "Thus you shall say to the sons of Israel, You yourselves have seen that I have spoken to your from heaven." Also in the New Testament at Mark 1:11, "a voice came out of the heavens; "Thou art My beloved Son, in Thee I am well-pleased."
The simple answer is no.
God lives in eternity, so He knows everything always (i.e. in what's human's "past, present, and future"). God knew in advance that Abraham would pass the test and was ready to sacrifice Isaac; God knew He'd have to stop that.
However, He'd have to speak our word "now" so humans and Scripture readers would understand the story better.
(Genesis 22:11-14, DRB) 11And behold, an angel of the Lord from heaven called to him, saying: Abraham, Abraham. And he answered: Here I am. 12And he said to him: Lay not thy hand upon the boy, neither do thou any thing to him: now I know that thou fearest God, and hast not spared thy only begotten son for my sake. 13Abraham lifted up his eyes, and saw behind his back a ram, amongst the briers, sticking fast by the horns, which he took and offered for a holocaust instead of his son. 14And he called the name of that place, The Lord seeth. Whereupon, even to this day, it is said: In the mountain the Lord will see.
Reading the verse in its proper context, there is some ambiguity regarding the speaker of the bolded text. Either the speaker is the angel, or it's God.
- If the speaker is the angel, then the verse tells us nothing about the omniscience of God and there isn't even an apparent contradiction.
- If the speaker is God, the bolded text still poses no challenge to God's omniscience because OP's argument depends on the fallacy "now" = "now and never before."
Example why "now" ≠ "now for the first time"
I'm using the computer now. I've actually been on it for more than an hour.